There are many ways of forming an image. Images can be formed through thermal transfer of dyes, inkjet applications, electrophotographic reproduction, and silver halide image development. Also known is that all such images are susceptible to environmental factors, particularly light fade. Thermal, inkjet, and electrophotographic images also can suffer from iridescence problems, which are unsightly to the viewer. Typically, iridescence is caused by the interaction between the materials on the receiver and any materials applied to the receiver in forming the image.
To form any printed image, the image is either chemically developed from film, or developed from an electronic signal generated from either a digital capture device, or scanning of a film. For thermal, inkjet, and electrophotographic prints, electronic signals indicating appropriate colors are used to produce cyan, magenta and yellow color signals. These signals are then transmitted to a printer where colored material is transferred to a receiver. A color hard copy is thus obtained that corresponds to the original image.
Thermal, ink jet, and electrophotographic prints are susceptible to retransfer of colorants to adjacent surfaces and to discoloration by fingerprints because the colorants remain at the surface of the receiver. Heat can be used to drive the colorants deeper into the receiver. Application of a protective overcoat on these types of prints, as well as silver halide prints, is also known, and effectively reduces retransfer and discoloration by adding a protective polymeric layer over the image.
The protective overcoat can also provide improved light stability to the underlying imaging colorants, including dyes. The most common approach is to filter out UV radiation since it is known that UV radiation is detrimental to the underlying colorants. Improved image stability can be achieved with the addition of a UV absorbing dye in a protective overcoat, as described in U.S. Pat. No. 4,522,881. This approach has practical limitations on the amount of UV radiation that can be absorbed because there is a practical limitation on the thickness of the protective overcoat as well as the concentration of the UV absorbing dye that can be applied.
Improved image stability can also be achieved by incorporating light stabilizers in close proximity to the colorants within the receiver. Light stabilizers can be added to the receiver during manufacture by aqueous or solvent coating or thermal extrusion of materials incorporating the light stabilizers. If thermal extrusion is used, only light stabilizers with very high thermal stabilities can be used due to the temperatures of extrusion, typically 250° C. or higher. The light stabilizers must be incorporated within the receiver in such a manner that they will react with the colorants when applied to the receiver.
U.S. Pat. No. 5,332,713 discloses a transferable protection overcoat on a donor element for transfer to a thermal print. The transferable protection overcoat comprises poly(vinyl formal), poly(vinyl benzal) or poly(vinyl acetal) containing at least about 5 mole % hydroxyl. The overcoat provides inferior gloss and iridescence performance due to refractive index mismatch with the dye receiving layer.
U.S. Pat. No. 5,387,573 discloses a protective overcoat including particles in an amount of up to about 75% of the thickness of the heat transferable protective overcoat. Although the particles reduce the iridescence problems, the particles lower the gloss of the imaged print.
U.S. Pat. No. 5,670,449 discloses the use of elastomeric beads in a protective overcoat for better raw-stock keeping, but the gloss performance of these protective overcoats is not optimum.
U.S. Pat. No. 6,942,956 discloses a protective overcoat comprising a gloss-enhancing agent, and a mixture of inorganic and organic particles. In some embodiments, the protection layer contains from about 5% to about 60% by weight inorganic particles, from about 25% to about 80% by weight polymeric binder and from about 5% to about 60% by weight of organic particles, and an effective amount of at least one gloss-enhancing compound. The gloss enhancing compound consists of an organic molecule that is essentially colorless, does not scatter light, is substantially not absorbing of light at a wavelength from 400 to 800 nm, and has a maximum absorption at a wavelength less than 400 nm. The inorganic particles, e.g. silica, are required to provide smooth protective overcoat tear-off, but these degrade gloss and are detrimental to the gravure coating quality. The organic particles are required to reduce iridescence, but these reduce gloss. The gloss improvement provided is not adequate.
U.S. Pat. Nos. 7,301,012 and 7,384,138 disclose the use of hindered amine light stabilizers (HALS) in a receiver to provide image dye stability.
There remains a need for a heat transferable protective overcoat that provides greater image stability, reduced iridescence, and can be manufactured at low cost.